1. Field of the Invention
The present invention relates to continuous tone color thermal printers and, more particularly, to pulse count modulation for such printers.
2. Description of the Prior Art
Some thermal printer apparatus use a dye transfer process. In this process, a carrier containing a dye is disposed between a receiver, such as paper, and a print head formed of, for example a plurality of individual thermal heat producing elements often referred to as heating elements or pixels. The receiver and carrier are generally moved relative to the print head which is fixed. When a particular heating element is energized, it is heated and causes dye to transfer, e.g. by sublimation (from the carrier to an image pixel in the receiver). The density or darkness of the printed dye is a function of the temperature of the heating element and the time the carrier is heated. In other words, the heat delivered from the heating element to the carrier causes dye to transfer to an image pixel of a receiver. The amount of dye is directly related to the amount of heat transferred to the carrier.
Thermal dye transfer printer apparatus offer the advantage of true "continuous tone" dye density transfer. By varying the heat applied by each heating element to the carrier a variable dye density image pixel is formed in the receiver.
Continuous tone thermal printers are used to print color images. First a cyan image is printed and then magenta and yellow images are superimposed on the cyan image to form a colored image as shown in FIGS. 1-4 of this application. Any order of printing can be used. Pulse count modulation is often used to provide continuous tone thermal prints. The heat delivered is adjusted by changing the number of pulses applied to each heating element. As shown in FIG. 4, in such a system the pulses have a constant current at a fixed width. In FIG. 4 for purposes of illustration there are only a maximum of four possible pulses. Only one pulse is activated, then the image pixel will have 25% of the maximum density. If two pulses are activated it will have 50%. If three pulses are activated it will have 75% and if all four pulses are activated it will have 100%. Let's assume for a more practical example that the maximum dye density pixel receives 200 pulses. There are then 201 discrete dye density levels since a heating element may receive anywhere between 0 and 200 pulses. U.S. Pat. No. 4,806,949 to Onuma et al discloses a pulse count modulation scheme for printing a multiple tone image with two signals. In this disclosure, the first signal corresponds to the least significant bit of the digital tone data. The second signal is comprised of a series of pulses, each pulse being twice as long in duration as the first signal. The number of pulses in the second signal corresponds to the digital tone data, excluding the least significant bit. The addition of the first signal effectively doubles the number of energy levels (corresponding to density levels) achieved by the pulses in the second signal.